The present invention relates to an exposure apparatus for use in a photolithographic process for fabricating semiconductor devices, liquid crystal displays, image sensors, such as CCDs, thin-film magnetic heads or others. More particularly, the present invention relates to a technique suitably applying to a scanning exposure type of exposure apparatus such as a so-called slit-scanning type or step-and-scanning type exposure apparatus, with which a pattern shaped on a mask is sequentially transferred by exposure onto a photosensitive substrate by synchronously scanning the mask and the photosensitive substrate in respect to an illumination area that is, for example, a rectangular or an arc in shape.
Conventionally, the exposure apparatuses had been used for fabricating semiconductor devices, etc. are one-shot exposure type of exposure apparatuses such as steppers, etc. with which a pattern shaped on a reticle (or a photomask) is transferred to each shot area on a wafer (or a glass plate, etc.) on which photoresist is applied, through an optical projection system by a exposure method of step-and-repeat type. Such type of the exposure apparatuses have an illuminance control unit by which quantity of exposure (i.e., exposure energy) to each shot area on the wafer can be controlled within appropriate range.
FIG. 11 shows a conventional stepper-type of projection exposure apparatus which has an illuminance control system. In FIG. 11, an illumination beam emitted from a mercury-vapor lamp 101 is gathered by an ellipsoidal mirror 102 and then enters a shutter 104 through a condenser filter system 103. The condenser filter system 103 comprises a condenser optical system 103a and an optical filter 103b through which a light having a desired wave-length band, e.g., i-rays, selectively passes. The shutter 104 is opened or closed by a shutter control unit 105 according to an order from a timer control system 106. The shutter 104 is operable to open and close by means of a shutter control system 105 based on an order from a timer control system 106. When the shutter 104 is open, the illumination beam is substantially collimated by an input lens 107 to reach a fly-eye lens 108.
The fly-eye lens 108 forms a plurality of images of the light source at an exit surface thereof so that illumination distribution on a reticle 119 of the illumination beam becomes uniform. A turret plate 122, on which a plurality kind of an illumination-system-aperture-stop are formed, is disposed near the exit surface of the fly-eye lens 108. The illumination beam which passes through a predetermined illumination-system-aperture-stop enters a mirror 109 which has a reflectivity of about 98%. The illumination beam reflected by the mirror 109 passes through a first relay lens 113 and then a part of the illumination beam reflected is limited to a constant illumination area on a blind (a variable field stop) 114. The illumination beam passing through the blind 114 is relayed by a second relay lens 116, a mirror 117 and a condenser lens 118 so as to illuminate an illumination area on the reticle 119 with uniform illumination distribution. By this unified illumination beam, a pattern on the reticle 119 is transferred to each shot area on a wafer 121 by exposure through a projection optical system 120 with, for example, the reduction rate of one fifth.
When the illumination beam enters the mirror 109, a leakage light passing through the mirror 109 enters an integrator sensor 111 comprising a photodetector through a condensing lens 110. The integrator sensor 111 emits output signal corresponding to illuminance of the leakage light and the output signal is provided to an illuminance calculation system 112. The photosensitive surface of the integrator sensor 111 is positioned in a plane optically conjugate to a plane of, for example, a pattern bearing surface of the reticle 119. A scaling factor giving a relation between the exposure energy per unit time of the illumination light irradiated to the wafer 121 and the illuminance on the integrator sensor 111 is memorized in advance. At the illuminance calculation system 112, by calculating a product of a strength of the output signal emitted from the integrator sensor 111 and the scaling factor, the exposure energy per unit time of the illumination light irradiated to the wafer 121 is gained. The data with respect to the exposure energy per unit time is provided with a main control system 125 in which exposure time is gained by dividing the appropriate exposure to be irradiated onto the wafer 121 by the exposure energy per unit time. The exposure time is inputted to the timer control system 106. The shutter 104 is controlled by the timer control system 106 so as to be open only for the exposure time so that the accumulated exposure on the wafer 121 is controlled to be equal to the appropriate exposure.
Recently, in order to raise resolution and focal depth when a pattern on a reticle having a small and cyclical pitch is transferred by exposure, the modified illumination method (refer to Japanese Patent Disclosure No. HEI 4-225358 (1992/225358), corresponding to U.S. patent application Ser. No. 791,138 filed on Nov. 13, 1991) in which an illumination-system-aperture-stop has a plurality of apertures eccentrically shaped with respect to an optical axis, the annular illumination method in which the illumination-system-aperture-stop has a circular zone and etc. are proposed. In the apparatus shown in FIG. 11, also, the aperture stop for the modified illumination method and the aperture stop having the circular zone for the annular illumination method are shaped in the turret plate 122. When the main control system 125 supplies information about a pattern to be transferred by exposure to an illumination-system-aperture-stop-control-system 124, the turret plate 122 is rotated by a motor 123 so that the appropriate illumination-system-aperture-stop is selectively set to an exit surface of the fly-eye lens 108.
Thus, when the shape of the illumination-system-aperture-stop changes, the number and distribution of images of a light source in the illumination-system-aperture-stop change so that illuminance on the wafer 121 changes. In the one-shot exposure type of the exposure apparatus, even if the illuminance on the wafer 121 changes as described above, the exposure irradiated onto each shot area on the wafer can be always appropriately controlled by taking, for example, such a manner that an output signal from the integrator sensor 111 which has a photosensitive surface positioned in a plane optically about conjugate to a photosensitized surface of the wafer 121 is monitored so that the time for which the shutter 104 is open is controlled according to the output signal.
In the one-shot exposure type of the exposure apparatus, illuminance unevenness in a surface on the wafer to be irradiated is controlled by the fly-eye lens 108, etc. as an optical integrator. Also, because the shutter 104 is controlled by indirectly monitoring the accumulated exposure irradiated onto the wafer 121 with the output signal from the integrator sensor 111, stability with time of the illuminance in each shot area on the wafer 121 has not been questioned. Further, even if illumination condition in the modified illumination method, etc. between the integrator sensor 111 and the light source (or the mercury-vapor lamp 101) is altered, the time until which the accumulated exposure amounts to the desired value only changes and therefore there is nothing to be particularly inconvenient.
The prior art stated above is the one-shot exposure type of the exposure apparatus in which the pattern is transferred by exposure to each shot area on the wafer by once open-and-shut-action of the shutter, respectively, with the reticle and the wafer being static with respect to a projection optical system and which has the illuminance control mechanism. In such one-shot exposure type of the exposure apparatus, there is nothing to be particularly inconvenient on control of illuminance.
Meanwhile, recently, an one chip pattern for a semiconductor and the like has come to have a larger and larger size so that the projection exposure apparatus is required to have a larger size for efficiently projecting a pattern with a larger area onto the wafer. To project the pattern with the larger area onto the wafer, it will be particularly necessary to control distortion in the entire surface of the wafer so as to be lower than a predetermined value. Therefore, to decrease the distortion in the entire surface of the wafer and project the pattern with the larger area onto the wafer, a scanning exposure type of projection exposure apparatus, for example, a so-called step-and-scanning type or slit scanning type of projection exposure apparatus has been reconsidered. In the scanning exposure type of projection exposure apparatus, the reticle and the photosensitized plate are synchronously scanned with respect to a radiating region which has a shape of, for example, rectangular, circular arc, a plurality of trapezoid and etc. (hereinafter called a slit radiating region) so that the pattern on the reticle is successively transferred to each shot area by exposure.
In case of applying the method of controlling the accumulated exposure of the prior art as described above to the scanning exposure type of projection exposure apparatus, various inconveniences occur. First, in the scanning exposure type of projection exposure apparatus, the accumulated exposure irradiated in each shot area is controlled by such a manner that the accumulated exposure in a slit-like exposure field is maintained at a constant at all points of the wafer. If the accumulated exposures at each points of the wafer are different from one another, unevenness of the accumulated exposure arises. This unevenness results in the same errors as the illuminance unevenness in the surface on the wafer to be irradiated when the one-shot exposure type of the exposure apparatus is used.
A constant-illuminance-control-method is known as a method for avoiding this unevenness. This is such a method that the output from the integrator sensor is directly returned to a power supply source and then illuminance on the wafer is controlled based on the output so that the illuminance on the wafer is maintained at a constant. However, the exposure, or the absolute exposure, irradiated per unit time to the entire exposure field by the scanning exposure method is larger than that by the one-shot exposure method because an illumination area in the scanning exposure method is narrower than that in the one-shot exposure method. It will be necessary to make the variable range of the exposure larger because it is necessary to vary the accumulated exposure within a large range according to a photographic sensitivity of a resist.
However, when the variable range of the exposure is made larger, if SN ratio of the output from the integrator sensor is intended to be maintained at a value larger than the predetermined value when both the quantity of light irradiated is large and that is small, a lot of quantity of light (in case of continuous light beam, illuminance or the light energy per unit time of the entire light beam irradiated) is irradiated to the integrator sensor. Therefore, there is such an inconvenience that temperature of the integrator sensor changes when measuring the quantity of light so that sensitivity of the sensor changes. Further, the quantity of light irradiated onto the wafer stage in the scan exposure method is larger than that in the one-shot exposure method. Therefore, when the illumination light with the quantity of light over a wide range is irradiated to an illuminance unevenness sensor which is a photoelectric sensor on the wafer stage and an exposure monitor which is a photoelectric sensor for measuring the quantity of light reflected by the wafer, a change in temperature which cannot be disregarded occurs so that the change in temperature causes an inconvenience such that a sensitivity of the sensor changes.
In view of the above-described problems, an object of the present invention is to provide an exposure apparatus such that an error when the quantity of light is measured with a sensor for measuring the quantity of light is small and an output signal having a high SN ratio is gained even if the quantity of irradiation changes within a wide range, and the quantity of irradiation or the accumulated exposure can be measured with high-accuracy.